Device and method for positioning a target volume in radiation therapy apparatus

ABSTRACT

Device for positioning a target volume ( 112 ) such, as a phantom or a patient in a radiation therapy apparatus, said apparatus directing a radiation beam ( 405 ) towards said target ( 112 ), characterized in that it comprises:—a target support ( 100 ) whereon the target is immobilized; a two dimensional radiation detector ( 103 ) fixed with fixations means ( 101, 102, 104, 106 107; 301, 302, 304, 305, 306; 208, 209 ) in a known geometric relationship to said target support ( 100 ), said radiation detector ( 103 ) being capable of detecting the position of intersection of said radiation beam ( 105 ) with said detector ( 103 ); correcting means for correcting the relative position of said beam ( 105 ) and said target support  100 ), based on said detected intersection position.

TECHNICAL FIELD

The invention relates to the field radiation therapy. More particularly,the invention relates to a device and method for positioning a targetvolume in a radiation therapy apparatus. Said target can be a patient ora phantom representing said patient for irradiation tests.

DESCRIPTION OF RELATED ART

When a tumour has been discovered in a patient's body, the tumour firstneeds to be visualized for further diagnostic. This can for example beperformed with an imaging system such as a computed tomography (CT)scanning system or an MRI apparatus. A 3D representation of the tumourvolume is obtained. With the information collected in this way aclinical treatment plan can be generated. Then the actual treatment witha radiation therapy apparatus can start.

In a known hadron therapy apparatus, like e.g. used in proton therapy,the therapeutic radiation beam, accelerated by an accelerator such as acyclotron, is guided to the therapy room, where the patient is fixed toa therapy table such as a therapy couch. Prior to irradiation, thetherapy table must be positioned accurately by means of a positioningsystem, such that the affected part of the patient's body is inalignment with the therapeutic radiation beam.

Often irradiation is performed from a variety of different angles. Agantry system is then used to rotate the radiation source. For hadrontherapy, typically, the hadronic particle beam, e.g. a proton, aneutron, an alpha particle or a carbon ion beam, is emitted out of anozzle towards a particular target region of the patient, e.g. a tumourto be treated. Tumour location are determined in an imaging system(CT-scan or MRI), together with/through markers. These markers may benatural bony landmarks, visible in the diagnostic imaging system, orartificial markers, such as gold seeds inserted (under the skin) in thepatient, or marks made on the skin with a radioopaque ink, in case ofCT-scan imaging. It is a requirement that all these types of markers arevisible under both the diagnostic imaging system used and the systemused for positioning the patient in the radiation therapy apparatus.

In a gantry radiation therapy apparatus, the isocentre is defined as thepoint where the axis of the nozzle beam path intersects the gantry axisof rotation. The isocentre remains fixed throughout the treatmentprocess.

It is required that the patient be accurately positioned with respect toradiation therapy apparatus, so that the beam only irradiates the targetas designed in the treatment planning. Otherwise, the beam could damagehealthy cells within the patient's body. So, the precision of thepatient positioning before treatment is highly critical to a successfuloperation.

Typically, the patient undergoing the therapy receives periodictreatments wherein the target is irradiated repeatedly over the courseof an extended period of time. Therefore, before each treatment session,a proper positioning of the patient is required.

As already mentioned, the process of identifying the precise location ofthe target volume immediately prior to a dose of therapeutic radiationis of key importance. To ensure that the patient is accuratelypositioned with respect to the therapy device, the position of thetarget is initially determined with respect to one or more monumentswithin the body of the patient. This operation is performed in anoutside diagnostic imaging system, such as a PET-scanner. In standardpatient positioning, the monuments are comprised of points on the bonesstructure of the patient and the location of the target is thendetermined with respect to these monuments. The patient is then locatedon the couch of a patient positioner in the therapy device. The patientpositioner is moved to the so-called ‘set-up position’, where thepatient position is determined through the use of radiographs,laser-lights and/or other localization aids having the capability tolocate said monuments. A correction is determined as required. Thepatient positioner is then used to move the patient to the successivetreatment positions. In the set-up position, the patient is located ator near the isocentre. In this approach it is assumed that the tumournever moves between the time of the CT scan and all the treatment days.

PRIOR ART DISCUSSION

U.S. Pat. No. 5,117,829 discloses a patient alignment system andprocedure for radiation treatment.

The method disclosed therein comprises the steps of

-   -   obtaining a 3D image of the region to be treated, together with        monuments;    -   compute from said 3D image a digitally reconstructed radiograph        (DRR) representing the image that would be obtained from said        region to be treated and monuments from an imaging system        located in the therapy device;    -   obtain a digital radiograph (DR), including location of        monuments;    -   compare location of monuments on DRR and DR, and compute a        correction for the position of the patient from this comparison.

The DR may be obtained in from an anterior/posterior (AP) or lateraldirection, for different types of corrections. In the case of an APimage, a X-ray source must be inserted into the nozzle, for directing animaging beam along the direction of the treatment beam, and removedbefore treatment beam delivery. This method however, assumes that therelative geometry of the imaging device or devices in the therapy devicewith respect to the treatment beam as delivered is known in a reliableway. In this and similar patient positioning methods, a precise andrepeatable mechanical patient positioner is required, having means forcorrecting as well translational as angular coordinates.

Many of the known methods for patient positioning in a radiation therapyapparatus have been designed with the application to a gantry radiationtreatment apparatus in mind. An example of such a method and device isthe above discussed U.S. Pat. No. 5,117,829. In this document, a targetisocenter location is selected and marked that identifies the particularlocation within the treatment volume at which the treatment beam shouldbe directed. The radiation therapy apparatus isocenter is that point 90in the center of the gantry at which the beams converge regardless ofthe position around the circumference of the gantry from which the beamsoriginate. It is thus the purpose of the treatment plan to bring thetissue volume of interest, i.e., the target isocenter, to a particularpoint in space having a fixed geometrical relationship relative to theradiation therapy apparatus isocenter 90. Typically, for many treatmentplans, the target isocenter will be brought directly to the radiationtherapy apparatus isocenter 90. However, radiation therapy devices witha radically different principle have also been designed. Patent documentWO 2005/053794 describes such a design, where a so-called “excentric”gantry carries a beam away from a rotation axis of said gantry towardstwo or more distinct treatment rooms. Thanks to an appropriate shieldingdesign, a patient may be treated in one of these treatment rooms whileanother patient is being prepared in another such treatment room. Theconcept of isocenter is absent in such a device.

There is therefore a need for a device and method for positioning apatient in a radiation therapy apparatus providing an improved accuracyand reliability, that must be applicable in a device lacking anisocenter.

SUMMARY OF THE INVENTION

The present invention overcomes the shortcomings of the conventional artand may achieve other advantages not contemplated by conventionaldevices.

According to a first aspect of the invention, a device for positioning atarget such as a phantom or a patient, in particular the tumour of apatient to be treated, in a radiation therapy apparatus, said apparatusdirecting a radiation beam towards said target, is disclosed. Saiddevice advantageously comprises:

-   -   a target support whereon the target is immobilized;    -   a two dimensional radiation detector fixed with fixation means        in a known geometric relationship to said target support, said        radiation detector being capable of detecting the position of        intersection of said radiation beam with said detector;    -   correcting means for correcting the relative position of said        beam and said target support, based on said detected        intersection position.

The device comprises a first set of ionization chambers in a firstorientation and a second set of ionization chambers in a secondorientation, thereby allowing determination of intersection point. Amatrix device can also be used.

The fixation means can be fixation frames affixed to the target support,e.g. in indexed fixed holes, whereby said detector may be located inclose proximity to the target. By having the detector located close tothe target and in particular a tumour in the patient, a positioningprecision as high as the resolution in the detector may be obtained.

The fixation means can also comprise fixation frames affixed to targetsupport with a slidable carriage sliding on rails whereby said detectormay located in close proximity to the target and in particular thetumour in the patient, in a continuous manner.

The fixation frames may be are circular arcs or polygons.

Preferably, the radiation detector and radiation beam are arranged suchthat said radiation beam intersects said detector in a substantiallyperpendicular direction, thereby providing a good precision in theintersection point.

The correcting means may comprise means for applying a precisecorrection displacement to said radiation beam. This solution isespecially preferred in radiation therapy apparatuses having scanningmagnets for moving the beam. A correction offset is then simply appliedto the scanning magnets. The correcting means may also comprise meansfor applying a precise correction displacement to the target support.Said detectors are positioned in front of the target.

According to a second aspect of the invention, a method is provided forpositioning a target in a radiation therapy apparatus comprising thesteps of:

-   -   determining a relative position of the target and a radiation        detector;    -   providing means ensuring that said relative position remains        fixed in the subsequent steps;    -   determining the intended intersection point of the radiation        beam with detector;    -   directing a sub-therapeutic radiation beam towards detector        device and determining the actual intersection point of said        beam with said detector;    -   computing a correction vector being the difference between the        two locations;    -   applying a correction to the relative position between radiation        beam (105) and radiation detector (103), whereby the actual        intersection point coincides with said intended intersection        point. Said target can be a phantom for irradiation tests or a        tumour in a patient.

The application of a correction to the relative position betweenradiation beam and radiation detector preferably comprises the step ofmoving the radiation beam. Alternatively, the application of acorrection to the relative position between radiation beam and radiationdetector comprises the step moving together the target and the detector.

Preferably, the step of determining relative position of target anddetector comprises the step

-   -   providing a detector having three or more points for fixation to        a target support, said points having a known geometric        relationship with said detector;    -   installing the target in a fixed way on target support;    -   providing markers on said target support at the fixation points        of said detector, said markers being visible in an imaging        modality;    -   obtaining an image of said target using an imaging modality        where both said target and said markers are visible;    -   determining from said image the geometric relationship of said        target and said markers;    -   inferring therefrom the geometric relationship between said        target and said detector.

According to a third aspect of the invention, the use of the deviceand/or the method according to the invention in a radiation therapyapparatus having an excentric gantry.

Other aspects and advantages of embodiments of the invention will bediscussed with reference to the figures and to the detailed descriptionof preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a radiation therapy apparatus using adevice according to the invention.

FIG. 2 represents a definition of the target support coordinate systemand of the nozzle coordinate system.

FIGS. 3, 4 and 5 represent features of a couch according torespectively, a first, a second and a third embodiment of the invention.

FIG. 6 is a schematic view of circular fixation frames.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS OF THE PRESENT INVENTION

FIG. 1 is a partial view of a radiation therapy apparatus where a deviceaccording to the invention is used. A radiation beam 105 is directedtowards a patient 111 lying on a patient support 100. The irradiationnozzle is shown symbolically as a deflecting magnet 113. According tothe invention, a radiation detector 103, shown in sectional view on FIG.1, is fixed to patient support 110 closely located to the patient 111and its tumour 112, in such a way that beam 105 intersects detector 103under a substantially perpendicular direction.

Referring to FIG. 2, the general geometric framework for discussingpatient positioning in a radiation therapy apparatus is described. Apatient (not represented) is lying on a couch 100 or other patientsupport. The patient coordinate system (Xp, Yp, Zp) has its origin at areference point in the tumour. The Yp axis is parallel to the long axisof the couch 100. The Xp axis is parallel to the short axis of the couch100. The Zp axis is vertical. The treatment beam 105 is delivered by anozzle 110. A coordinate system (Xn, Yn, Zn) is attached to the nozzle.The axis Zn is collinear with the treatment beam. The axis Xn and Yndefine the orientation of the aperture placed on the nozzle.

The position of the treatment beam 105 is determined by the followingconditions:

-   -   1. The treatment beam 105 must intersect the centre of the        patient coordinate system (as the reference point in the tumour        is at the centre of the patient coordinate system)    -   2. The distance D between the origin of the patient coordinate        system and the nozzle is defined by the Treatment Planning        System. It is not critical that this distance is measured        accurately. Indeed, the range of the particles in the patient is        related to the thickness of the tissues. If the distance D is        too large (or too short), this implies that the air gap between        the patient and the nozzle is too large (too short,        respectively). The thickness of tissues remains unchanged.        Therefore, increasing (decreasing, respectively) the air gap        will not have a significant effect on the range of the particles        in the patient.    -   3. The elevation angle β between the treatment beam and the (Xp,        Yp) plane is defined by the Treatment Planning Software (TPS).        This angle is related to the gantry angle.    -   4. The horizontal rotation angle α between the Yp axis and the        projection of the treatment beam in the (Xp, Yp) plane is        defined by the TPS. This angle is related to the couch rotation        angle.    -   5. The rotation of the (Xn, Yn) coordinate system around the        radiation beam is defined by the angle γ that is computed by the        TPS. This angle defines the rotation of the beam aperture with        respect to the patient. For symmetrical beams, this angle is not        important.

In conclusion, six parameters are required to specify the relativeposition of the Patient Coordinate System (Xp, Yp, Zp) and of the NozzleCoordinate System (Xn, Yn, Zn):

-   -   Three coordinates defining the position of the origin of the        patient coordinate system in the nozzle coordinate system. These        coordinates are defined by conditions (1) and (2)    -   Three angles (α, β, γ) defining the relative orientation of the        patient coordinate system and of the nozzle coordinate system.        These angles are defined by conditions (3), (4) and (5).

The relative position and orientation of the patient and nozzle has beendefined by 6 parameters expressed in the coordinate systems defined inFIG. 2. It is obvious that other coordinate systems can also be used.For example, instead of using the angles (α, β, γ) as defined in FIG. 2,one could define the rotation, pitch and roll angle of the couch.However, whatever coordinate system is used, one will always need todefine three angles and three position coordinates. A transformation canbe used to convert from one coordinate system to another. Therefore, wewill base our discussion on the coordinate systems presented in FIG. 2without loss of generality.

In the positioning devices and methods known in the art, an imagingsystem, comprising an imaging beam source and an imaging beam receiverhaving a known geometric relationship with respect to the nozzlecoordinate system is directed at the patient. Said imaging systemidentifies the location of visible landmarks which have known geometricrelationships with respect to the tumour reference point. From thedifference between the expected location of these landmarks and thedetected locations, a correction vector is computed.

In the framework of the invention, we introduce a new coordinate system.Let us suppose that a measurement plane 120 is placed in the treatmentbeam 105. The beam profile is measured in this plane. Ideally, thisplane is perpendicular to the treatment beam. The measurement planedefines two coordinate axes: Xd, Yd. Three angles (αd, βd, γd) definethe relative orientation of the measurement plane coordinate system withthe nozzle coordinate system.

During the treatment, the angular orientation (α, β, γ) of the patientwith respect to the nozzle is set by selecting the proper couch rotationangle, couch pitch and roll angles and gantry rotation angle. Thecorrect angular orientation of the couch and gantry rotation obtainedthrough the knowledge of the geometry of the treatment room environment.These positions may in addition be checked using known technologies (forexample, using stereo cameras to check the position of markers on thenozzle and on the patient or on the couch, such as described, e.g. indocument US 2002/0065461).

Once the angular orientation of the patient has been checked, thetranslational position of the patient must be verified. As previouslymentioned, the position along one of the translation direction (the Znaxis) is not critical. The present invention describes a method to checkthe relative position of the patient and of the treatment beam along thetwo remaining translation directions: the Xn and Yn axis.

The patient will be lying on the couch 100 (see FIG. 3). Two series offixation holes 107 are provided on the couch 100. These holes areindexed and are used to attach a patient immobilization device to thecouch 100. This provides a good reproducibility for positioning thepatient on the couch 100. A plane radiation detector 103 determines thespatial distribution in two dimensions of the intensity of the radiationbeam 105. The radiation detector is able to measure the intensitydistribution in two dimensions in a plane substantially perpendicular tothe axis of the radiation beam 105. This radiation detector may be a setof two strip ionizations chambers, with a first strip ionization chamberin one orientation and a second ionization chamber in anotherorientation. The radiation detector 103 may also be a pixel ionizationchamber. Other types of radiation detectors may be used, such assemiconductor or film. A selection of a radiation-transparent radiationdetector is preferred because a such radiation detector may be left inplace during the treatment. The value of the angle between the plane ofthe radiation detector 103 and the radiation beam is known.

At least 3 position markers 108 are installed on the couch 100 or on thepatient. The position markers 108 are manufactured with a material thatis visible both in a CT scanner and with a patient alignment systeminstalled in the treatment room (e.g. stereovision infra-red camera oran X-ray imaging system). These markers are known in the art from e.g.U.S. Pat. No. 6,865,253.

Two frames are attached to the couch. Several types of frames assemblyare possible: in a first embodiment, the frames are shaped as halfcircles (FIG. 3). The two frames 101 and 102 are fixed to the couch 100using the fixation holes 107. The fixation holes 107 used to attach theframes 101 and 102 are selected so that the distance between theapproximate position of tumour in the patient and the first frame 101 issubstantially equal to the distance between the tumour and the secondfixation frame 102. Therefore, the tumour is approximately in the middlebetween the two fixation frames. The half circle frames can be attachedto the top of the couch if the particle beam is coming from above thecouch. Alternatively, they can be fixed to the bottom of the couch ifthe particle beam comes from under the couch. FIG. 3 represents only theconfiguration when the proton beam is coming from above the couch. Thereare fixation holes 401 equidistantly drilled on the rim of the circularfixation frame 101 and 102 (see FIG. 6). These fixation holes areindexed. The three fixation legs 104, and 106 are inserted in thefixation holes 401. The radiation detector 103 is inserted on thefixation legs. The three fixation holes 401 and the length of thefixation legs 104 and 106 are chosen so that the radiation detector 103is measuring the radiation field in a plane that is substantiallyperpendicular to the radiation beam. The fixation legs will be specificfor a patient. A mechanism is implemented in order to make sure that thecorrect fixation holes 401 are used (FIG. 6). A plastic strip 402 isplaced on the rim of the fixation frame 101 or 102. There are two holes403 drilled in the plastic strip 402. There is one hole drilled on theplastic strip that will cover the fixation frame 102. The plastic stripwill cover all the holes 401 of the rims, except those that will be usedto fix the legs 104 and 106 of the radiation detector 103. The plasticstrip 402 is equipped with two fixation systems 410 and 411 that willattach the plastic strip to the fixation points 405 and 406 on the rimof the fixation frames 101 and 102. The fixations systems 410 and 411are asymmetrical, so that there is only one possible position in whichto place the plastic strip 402 on the rims. The fixation systems used onframes 101 and 102 will be different from the fixation system used onframe 102 in order to avoid inverting the plastic strips of these twoframes. The position of the holes 403 is carefully determined so thatthe measurement plane radiation detector will be positionedsubstantially perpendicular to the radiation beam. The position of thehole will therefore be different for each patient. A label 412 is placedon the plastic strip 402 in order to uniquely link the plastic strip andthe patient. The label could, for example, be a bar code. The fixationlegs 104 or 106 are inserted in the holes 401. The markers 407 areplaced at the end of the legs 104 and 106. These markers simulate theposition of the radiation detector. The markers can be visualized in aCT-scanner. The circular shape of the fixation frame 101 and 102 and theequidistance between the fixation holes 401 allows the fixation of theradiation detector 103 in a large range of angles around the patient.The angular range is covered by discrete angle steps.

In a second embodiment, the frames are shaped as half circles (FIG. 4).Two rails 208 are located on both sides of the couch 100. The two halfcircle frames 101 and 102 are attached to a carriage 209 that slides onthe rail 208. The position of the carriage on the rail is indexed sothat it is possible to reproducibly move the half circle frame to aspecified position. The carriage is moved so that the tumour isapproximately located in middle between the two fixation frames 101 and102. The radiation detector is fixed to the frames 101 and 102 asdescribed in Embodiment 1. The plastic strips 402 are used as describedin Embodiment 1.

In a third embodiment, the frames have a non circular shape, for examplethey can have a trapezoidal shape (FIG. 5). The two trapezoidal frames301 and 302 are fixed to the couch 100 using the fixation holes 107.There are two fixation holes on each side of the trapezoidal fixationframe 301. There is one fixation hole on each side of the trapezoidalframe 302. The legs 304, 305 and 306 of the radiation detector 103 areinserted in the fixation holes. In this embodiment, the radiationdetector can be at three different positions around the patient.

The method of patient positioning comprises two steps: a first step ofcalibration of the radiation detector prior to the treatment, and asecond step of actually positioning the patient during treatment

The first step, calibration of the radiation detector, is describedhereafter: When a new radiation detector is used, it must be calibrated.Indeed, the radiation detector measures the distribution of theradiation intensity in the measurement plane. The intensity distributionis described within the coordinate system of the radiation detector (Xd,Yd in FIG. 2). The position of the origin and the orientation of theaxis of this coordinate system will be different for each radiationdetector. Furthermore, the relative position of the origin of thecoordinate system and of the markers 407 will be different for eachdetector. In addition, the orientation of the coordinate axis of theradiation detector 403 with respect to the position marker 407 will bedifferent for every radiation detector. The calibration consists infinding the position of the origin of the radiation detector coordinatesystem with respect to the markers 407. It also consists in finding theorientation of the axis of the radiation detector coordinate system withrespect to the markers 407. This calibration process is done using knownprocedures. This calibration must be done for every new radiationdetector.

The second step, of actually positioning the patient comprises thefollowing steps:

-   -   1. a CT scan of the patient is acquired in a CT scanner;    -   2. using a Treatment Planning Software, the operator identifies        the position of the tumour, determines the incidence angles of        the treatment beams and prepares the treatment plan;    -   3. when the incidence angles of the different treatment beams        are known, the operator determines which treatment room of the        excentric gantry will be used for each treatment beam. She also        determines optimum position of the radiation detector 103 on the        fixation frames 101 and 102, which fixation holes 401 of the        fixation frame 101 and 102 will be used and the optimum length        of the fixation legs 104 and 106. The parameters are selected so        that the measurement plane of the radiation detector is        substantially perpendicular to the treatment beam. The plastic        strips 402 are prepared with the holes 403 drilled at the        correct position.    -   4. On the treatment day, the patient lies on the couch 101. The        operator places the fixation device on the patient in order to        install the patient at approximately the correct position on the        couch. The operator installs the fixation frames 101 and 102 on        the couch. The operator installs the plastic strips 402 for this        patient. The operator installs the fixation legs 104 and 106 on        the fixation frames. The operator places the markers 407 on the        fixation legs.    -   5. A CT scan or a CBCT scan of the patient, of the marker 108        and the markers 407 is acquired. Using positioning software, the        operator determines the relative position of the tumour and the        position of the markers 407. The operator also determines the        orientation of the patient coordinate system with respect to the        markers 108 and the position of the tumour with respect to the        markers 108.    -   6. Based on the position of the markers 407 and the calibration        procedure described above, the software determines the position        and orientation of the detector coordinate system with respect        to the patient coordinate system. Based on the knowledge of the        incidence angle of the treatment beam in the patient coordinate        system, the position of the tumour and the position of the        measurement plane, the software determines the point of        intersection of the radiation beam with the measurement plane.        The theoretical coordinate of the intersection point 501,        expressed in the radiation detector coordinate system, is where        the centre of the radiation beam must be directed.    -   7. The patient remains immobile on the couch. The couch is moved        from the CT scanner to the treatment machine. The patient is        placed in front of the nozzle delivering the treatment beam so        that the tumour is approximately placed on the axis of the        radiation beam.    -   8. The angular orientation of the patient with respect to the        treatment beam is verified by measuring the position of the        markers 108 using known technologies (e.g. stereovision        infra-red camera). The patient angular orientation is adjusted        by varying the pitch, roll and rotation angle of the couch until        the measured angular orientation of the markers 108 is correct.        The patient angular orientation is now correct.    -   9. The operator must now adjust the relative translation        position of the patient and the beam along the Xn and Yn axis of        FIG. 2. The theoretical coordinates of the intersection point of        the treatment beam with the radiation detector plane 103 has        been computed at step 6. A particle beam 500 with        sub-therapeutic intensity is aimed at the patient, along a        trajectory identical to that that will be used by the        therapeutic beam. The beam intensity is low enough not to cause        any damage to the patient while being high enough to be        detectable by the radiation detector 103. The relative        translation position of the patient and the treatment beam is        adjusted using the sub-therapeutic beam 500 using one of the        following method:        -   a. The couch is moved along an axis parallel to the (Xn,Yn)            plane until the sub-therapeutic beam 500 intersects the            detector plane at the theoretical position 501. This is the            preferred alignment method for the double scattering mode.        -   b. The sub-therapeutic beam is shifted transversally in the            (Xn, Yn) plane until it intersects the detector plane at the            theoretical position 501. A DC voltage is applied to the            beam scanning magnets in order to shift the sub-therapeutic            beam. This is the preferred method for uniform scanning and            pencil beam scanning.

All these steps are performed either by a technical operator or by acomputer through a specialized software calculator under the control ofa technical operator.

Finally, only when all these steps are performed, a final step, e.g.irradiating the patient, which is a therapeutic step and which is notpart of the present method, is performed.

In case of irradiation tests, a phantom is submitted to the irradiationbeam. In case of therapeutic treatment, only once the patient positionand the sub-therapeutic beam position have been adjusted as described instep 9, the same settings are used for the treatment beam. Only thislast step is made under the control of a doctor.

Finally, it should be noted that the detectors are preferably positionedin front of the patient or of the phantom. This means that during thetreatment of the patient through the irradiation beam, these detectorsmust be either transparent or removed from their position.

By using the device and method of the invention, an improved accuracy inthe positioning of a patient with respect to a therapeutic beam isobtained. This device and method is particularly, but not exclusively,suited for use in a radiation therapy device having an excentric gantry.The radiation detector being close to the patient skin, the error inpositioning of patient with respect to the beam can be as small as theresolution of the radiation detector. By using the variation of themethod and device where the position correction is applied by applyingan offset to the beam, the complexity and cost of a precise and patientpositioner may be avoided. The nominal position of the patient supportis not a critical factor in the solution of the invention. Moreover, inthe framework of the invention, one determines the position of theactual beam with respect to the patient, and not of the beam deliverydevice. One is thereby relieved from any errors in the alignment of thetreatment beam with respect to the nozzle.

The terms and descriptions used herein are set forth by way ofillustration only and are not meant as limitations. Those skilled in theart will recognize that many variations are possible within the spiritand scope of the invention as defined in the following claims, and theirequivalents, in which all terms are to be understood in their broadestpossible sense unless otherwise indicated.

DRAWING ELEMENTS LIST

-   -   100 target support (couch)    -   101 fixation frame (half circle)    -   102 fixation frame    -   103 radiation detector    -   104 fixation leg    -   106 fixation leg    -   105 treatment beam    -   107 fixation holes    -   108 position markers    -   110 nozzle    -   111 patient    -   112 target (tumour or phantom)    -   113 gantry magnets    -   120 measurement plane    -   301 trapezoidal frame    -   302 trapezoidal frame    -   304 leg    -   305 leg    -   306 leg    -   208 two rails on both sides of couch    -   209 carriage    -   401 fixation holes    -   402 plastic strips    -   403 radiation detector    -   403 holes    -   405 fixation point    -   406 fixation point    -   407 markers    -   410 fixation system    -   411 fixation system    -   412 label    -   500 sub-therapeutic particle beam    -   501 intersection point

1. A device for positioning a target in a patient in a radiation therapyapparatus, the apparatus directing a radiation beam towards the target,the device comprising: a target support for immobilizing the target; atwo-dimensional radiation detector fixed with fixation device in apredetermined geometric relationship to the target support, thetwo-dimensional radiation detector configured to detect an intersectionposition of the radiation beam with the detector; and correcting deviceconfigured to correct a relative position of the beam and the targetsupport, based on the detected intersection position; wherein thefixation device comprises fixation frames affixed to the target support,the two-dimensional radiation detector located in close proximity to thetarget.
 2. The device according to claim 1, wherein the fixation devicecomprises fixation frames affixed to the target support with a slidablecarriage sliding on rails, the two-dimensional radiation detectorlocated in close proximity to the target.
 3. The device according toclaim 1, wherein the fixation frames are circular arcs.
 4. The deviceaccording to claim 1, wherein the two-dimensional radiation detector andthe radiation beam are arranged such that the radiation beam intersectswith the two-dimensional radiation detector in a substantiallyperpendicular direction.
 5. The device according to claim 1, wherein thecorrecting device comprises an application device configured to apply aprecise correction displacement to the radiation beam.
 6. The deviceaccording to claim 1, wherein the correcting device comprises anapplication device configured to apply a precise correction displacementto the target support.
 7. The device according to claim 1, wherein thetarget support comprises indexed fixed holes for affixing the fixationframes.
 8. The device according to claim 1, wherein the fixation framesare polygons.
 9. A method for positioning a target in a patient in aradiation therapy apparatus, comprising: determining a relative positionbetween the target and a two-dimensional radiation detector; providing afixation device configured to fix the relative position during thesubsequent steps; determining an intended intersection point of theradiation beam with the two-dimensional radiation detector; directing asub-therapeutic radiation beam towards the two-dimensional radiationdetector and determining an actual intersection point of the beam withthe two-dimensional radiation detector; computing a correction vectorwhich is a difference between locations of the intended and the actualintersection points; and applying a correction to a relative positionbetween the radiation beam and the two-dimensional radiation detector,whereby the actual intersection point coincides with the intendedintersection point; wherein determining the relative position of thetarget and the two-dimensional radiation detector comprises: providing atwo-dimensional radiation detector having three or more points forfixation to a target support, the points having a geometric relationshipwith the two-dimensional radiation detector; installing the target in afixed way on a target support; providing markers on the target supportat fixation points of the two-dimensional radiation detector, themarkers being visible in an imaging modality; obtaining an image of thetarget using an imaging modality where both the target and the markersare visible; determining from the image the geometric relationship ofthe target and the markers; and inferring therefrom the geometricrelationship between the target and the two-dimensional radiationdetector.
 10. The method according to claim 9, wherein applying acorrection comprises moving the radiation beam to correct the relativeposition between the radiation beam and the two-dimensional radiationdetector.
 11. The method according to claim 9, wherein applying acorrection comprises moving the target and the two-dimensional radiationdetector to correct the relative position between the radiation beam andthe two-dimensional radiation detector.